Nickel isotope ratios trace the process of sulfide-silicate liquid immiscibility during magmatic differentiation

Abstract

Sulfide-silicate liquid immiscibility (i.e., the coexistence of two liquid phases in equilibrium) plays a significant role in the processes of planetary-scale differentiation, the formation of Earth’s crust, and the genesis of magmatic sulfide ore deposits. The stable isotope geochemistry of nickel can potentially take advantage of fractionation signals produced in magmatic systems where immiscible sulfide and silicate liquid have equilibrated. Variations in Ni isotope ratio beyond those found in sulfide-undersaturated basaltic rocks can provide a hallmark of fractionation processes that control chalcophile and siderophile element abundances. We report high-precision Ni isotope ratio data for a stratigraphically-controlled sequence of Siberian Trap basalts in a volcanic edifice centered over the world’s largest concentration of magmatic sulfide ore deposits at Noril’sk-Talnakh, Russia. Nickel isotope ratios (δ60Ni (‰) = [(60Ni/58Ni)sample/(60Ni/58Ni)SRM986-1] × 1000) in the basaltic rocks range from +0.07 ± 0.02‰ to +1.00 ± 0.04‰ and correlate negatively with Ni and precious metal abundance levels, indicating extensive Ni isotope fractionation due to sulfide saturation of the silicate magma. A Rayleigh fractionation model fits the observed results and enables a precise estimate of the fractionation factor (α) between sulfide liquid and silicate melt to be 0.99965 (i.e., 103*lnα = −0.35). Our study illustrates the application of Ni isotope ratio data in understanding sulfide-silicate liquid immiscibility and the broader implications with respect to the formation of the largest known magmatic sulfide ore deposits in association with the Siberian Trap.